High molecular compounds typified by plastics are abundantly produced and used around the world in a wide variety of fields for their convenient properties of lightness and strength.
However, used high molecular compounds generally do not degrade in the natural environment. Even if they are buried, they remain undegraded and accumulate year after year in the global environment. On the other hand, incineration of high molecular compounds causes problems such as occurrence of high heat and toxic gas. Accordingly, disposal of high molecular compounds has now turned to be a social problem.
Reflecting the ever-increasing public concern over environmental problems on the global scale in recent years, a great deal of research is now under way on biodegradable high-molecular substances which can be degraded by a microorganism in the natural environment and also on degradation of hardly-biodegradable polymers under the action of an enzyme or a microorganism. Some of this research is finding practical utility.
Biodegradable high-molecular substances which are now under research can be divided roughly into "synthetic high-molecular substances", "microbially-produced high-molecular substances" and "plant-or animal-derived, natural high-molecular substances".
Synthetic high-molecular substances permit molecular designing of high molecular substances having various functions from a large number of structural units and are expected to become substitutes for general plastics.
As a result of screening of polymer-degrading microorganisms, it has been found that water-soluble polyvinyl alcohol and polyethylene glycol can be degraded by certain microorganisms. Polyvinyl alcohol is degradable by a bacterium belonging to the genus of Pseudomonas (Suzuki et al., Agric. Biol. Chem., 37, 747, 1973). Further, polyethylene glycol whose molecular weight is 6,000 is degradable by a symbiotic bacterium system of a Pseudomonas sp. and a Flavobacterium Sp. (Kawai et al., J. Ferment. Technol., 55, 125, 1977). It has also been reported that water-insoluble solid aliphatic polyesters, especially hardly-degradable polycaprolactones are hydrolyzed in soil (Potts et al., Polym. Prepr., 13, 629, 1972).
Among aliphatic polyesters known as biodegradable high-molecular substances, polymers which are formed from lactic acid, glycolic acid, or lactic acid and glycolic acid have the features that they are more mold-resistant than natural high-molecular substances derived from plants or animals and also that they are superior in transparency to microbially-produced high-molecular substances and other synthetic high-molecular substances. Moreover, as the polymers formed from lactic acid, glycolic acid, or lactic acid and glycolic acid are hydrolyzed into lactic acid or glycolic acid, they have high biosafety and are thus suited for use in the field of medical materials or foods.
Properties required for a biodegradable high-molecular substance are high material strength during use but prompt degradation in the environment after disposal. Polymers formed from lactic acid, glycolic acid, or lactic acid and glycolic acid are known to be non-enzymatically hydrolyzable in the living body [Yamane et al., Jinko Zoki (Artificial Organs), 15, 1751, 1986]. However, their hydrolyzability decreases with the molecular weight. On the other hand, the material strength increases with the molecular weight. Accordingly, these properties contradict each other.
For example, poly(L-lactic acid) degrades in about 2 weeks in physiological saline when its molecular weight is 1,000. Its material strength is however so low that it is not practically usable. When the molecular weight increases to 10,000 or higher, its material strength increases to such a level as permitting its use as plastics although it becomes hardly degradable.
Several methods are known for the degradation of polymers formed from lactic acid, glycolic acid, or lactic acid and glycolic acid and having sufficient material strength. For example, it has been reported that the degradation speed of polylactic acid having a molecular weight of about 100,000 becomes higher when the polylactic acid is added with nutrients for a microorganism and is then buried in the earth (Japanese Patent Application Laid-Open No. 168150/1992). Even by this method, however, it takes about 3 months until the polymer degrades completely.
As a still further method for degrading a biodegradable high-molecular substance, it is also known that the biodegradable high-molecular substance may degrade when it is placed in a compost formed of sewage sludge, urban garbage, livestock excrements and the like. However, a compost is formed of various components and its form and colonies substantially vary depending on the place and also season in which the compost is produced. Therefore the method making use of a conventional compost cannot surely degrade a biodegradable high-molecular substance with good reproducibility, and in some instances, the biodegradable high-molecular substance may remain practically undegraded.
As has been described above, some examples are known to date regarding methods for degrading polymers by microorganisms. However, they are all accompanied by problems such as the need for a long time for the degradation of polymers and lack of reproducibility of degradation, so that they are not suited for industrial applications.
With the above-described conventional art in view, an object of the present invention is to provide a method for degrading a polymer having high strength at a time of use, which makes use of a solid phase with a microorganism, nutrients required for the growth of the microorganism and water retained on a carrier for promptly degrading the polymer with good reproducibility, and also an apparatus suitable for use in the practice of the method.